Pacific Giant Salamander Image Source: Gary Nafis1

Introduction

The Pacific Giant Salamander (Dicamptodon tenebrosus) is the largest terrestrial salamander in North America, growing up to 13 inches in total length 2. Pacific Giant Salamanders (subsequently reffered to as just “salamanders”) are born with gills. Some individuals retain these gills throughout their life for a more aquatic lifestyle or metamorphosize lungs, allowing for a more terrestrial lifestyle3. Salamanders are reported to bite when handled and even “growl” or “bark” when they feel threatened4. Pacific Giant Salamanders populations range from northern California to the Canadian border, but are absent from the Olympic Peninsula. Salamander habitats consist of wetlands such as permanent rivers, streams, creeks and pools in forests5. Salamander populations may be affected by increased pressures associated with logging, including habitat destruction and increased silt in water from upstream disturbances. This report analyzes differences in salamander populations observed in the H.J. Andrews Experimental Forest near Blue River, Oregon.

Map Source: Oregon State University6

Data and Methods

The data7 analyzed in this report was collected from Mack Creek in the H.J. Andrews Experimental Forest in Oregon. Cutthroat trout (Onchorhyncus clarki clarki) populations have been monitored since 1987. Observations of Pacific Giant Salamanders did not begin until 1993. The H.J. Andrews Experimental Forest contains two sections of clearcut forest (cleared in 1963) and an upstream coniferous forest that is 500 years old7. These two sections will be referred to as “clear cut”(CC) and “old growth”(OG) throughout this report.

Specimen were sampled via electro-fishing. Captured vertebrates were then measured and weighed. This report evaluates salamander data on forest section type (old growth or clear cut), channel classifications (cascades, pool or side-channel), salamander weight (in grams), and the total salamander counts observed (n).

Salamander abundance across locations was explored using statistical tests including chi-squared test, two-sample t-test, Cohen’s d, Levene’s test, one-way ANOVA, and Tukey’s test.

Results

A. Annual salamander counts in forest sections of Mack Creek

Figure 1 displays the overall trend of salamander population numbers. In general, salamander abundance is lower in clear cut areas compared to counts from old growth forest. Annual change in abundance for each forest section follows similar trends in growth and decline throughout the study period (1993-2017). However, in 2015 salamander populations in clear cut sections surpassed that in old growth. This shift lingered in the following years of the study.

B. Salamander counts in forest section and channel cascades of Mack Creek

Pacific giant salamander observations from 2017 were compiled to evaluate salamander counts and proportions in different channel classifications (pool, cascade, or side-channel) within Mack Creek for the two sections (old growth and clear cut).

Table 1 shows that the majority of salamanders observed in both clear cut and old growth forest sections were found in cascades (over 60% for both). Side channels contained the second most observations (between 23-24% of observations). The lowest proportion of salamanders were observed in pools (less than 15% of relative abundance). Overall, more salamanders were observed in clear cut than old growth sections–368 and 320 respectively.

Table 1. Counts and proportions for salamanders in channel classifications within different forest sections are shown in the table. Counts are shown as whole numbers, proportions are shown as percentages.
Section Cascades Pool Side-channel
Clear cut 247 (67%) 31 (8%) 90 (24%)
Old growth 201 (63%) 45 (14%) 74 (23%)

C. Impact of forest section on salamander counts in different channel classifications

A chi-square test was used to determine if there is an association between forest section (clear cut or old growth) and channel classification (cascades, pool, or side channel). The results indicate that forest section does not have a significant effect on salamander location, and are thus independent of each other (\(\chi\)2(2) = 5.54, p = 0.06).

D. Salamander weights in forest sections of Mack Creek in 2017

Pacific giant salamander observations in 2017 within clear cut and old growth forest sections have similar counts and distributions of salamander weight. The two sections of forest in 2017 have total salamander counts that differ by only 40 individual specimen. Therefore, it is important to compare statistics on salamander weight to find potential differences between the two sample populations.

PGS mean weight is 1.08 grams greater in clear cut sections than in old growth (7.78g and 6.7g, respectively). The results indicate that there is not a significant difference in mean weights for Pacific giant salamanders observed in the two forest sections (t(683.21) = (1.49, p = (0.138).

Large sample size, a lack of significance difference between sample means, and a small effect size (Cohen’s d = 0.11) indicate a lack of association between PGS weight and forest condition.

E. Salamander weights within channel classifications

Mean weights and sample size for PGS observations drawn from each channel classifications differ slightly. Cascades observed a mean weight of 7.52g among a sample of 448 salamanders. Pools observed a mean weight of 9.3g among a sample of 76 salamanders. Side-channels observed a mean weight of 5.68g among a sample of 164 salamanders (Table 2).

The channel classification with the highest mean weight (pool) was 3.62g greater than the channel with the smallest mean (side-channel). The channel with the largest count (cascade) contained 372 salamanders more than the channel with the lowest count (pool).

Table 2. Summary statistics for salamander weight among different channel classifications. Summary stats include mean, standard deviation, sample size, standard error, and variance.
Channel Classification Mean Weights Standard Deviation Sample Size (n) Standard Error Variance
Cascade 7.52 9.03 448 0.43 81.50
Pool 9.30 13.62 76 1.56 185.58
Side-channel 5.68 8.27 164 0.65 68.31

Variance in weight among channel classifications are somewhat similar. The channel with the largest variance is less than four times greater than that of the channel with the smallest variance. A Levene’s Test shows that variances are equal (because p-val >0.05). Therefore, it is appropriate to execute a one-way ANOVA to compare means between the three groups.

A one-way ANOVA indicates that the mean weights for the three classifications are not the same (F(2, 684) = 4.22, p = 0.015)). In response, a post-hoc pairwise comparison using Tukey is used to test which of the 3 channel classifications differ from one another (based on mean weight values).

A one-way ANOVA, paired with post-hoc Tukey’s HSD, reveals a significant difference only in mean salamander wieghts between salamanders found in side channels and pools (0.017). The weight of Salamander’s in cascades did not differ significantly from pools or side channels (0.286 and 0.084, respectively).

Weight observations for each channel classification were distributed in a relatively uniform manner around the mean. However, each channel classification contained multiple outliers that resulted in a skew of the mean values. Using a different summary statistic that is less affected by outlier values, such as median values, would allow for a more accurate depiction of this data (Fig. 2).

Summary

  1. On an annual scale, salamander abundance between different forest sections follow similar trends of growth and decline. However, prior to 2015, old growth sections continually experienced higher salamander counts than clear cut sections (Fig. 1).
  2. Channel type, rather than forest section, is a better indicator of salamander abundance. Table 1 shows the influence of channel classification resulting in a high proportion of salamander counts.
  3. In 2017, salamander distribution of weight and mean weight values were undifferentiated between clear cut and old growth sections (Fig. 2).
  4. Individual salamander weights in Mack Creek are bellow 25g, but outliers indicate that it is possible for salamanders to grow to a much larger weight (i.e., above 75g). Future studies should assess the factors limiting salamander growth (Fig. 3).

References

  1. “Dicamptodon tenebrosus; Pacific Giant Salamander.” Gary Nafis, California Herps, http://www.californiaherps.com/salamanders/images/dtenebrosusfchu3086.jpg

  2. “Coastal Giant Salamander - Dicamptodon tenebrosus.” California Herps, 2019, http://www.californiaherps.com/salamanders/pages/d.tenebrosus.html

  3. Dicamptodon tenebrosus, Coastal Giant Salamander, Animal Diversity Web, Mike Leighton, 2011, https://animaldiversity.org/accounts/Dicamptodon_tenebrosus/

  4. Pacific Giant Salamander, Coastal Giant Salamander, Amphibia Web, University of California, Berkley, 2019 https://amphibiaweb.org/cgi/amphib_query?where-genus=Dicamptodon&where-species=tenebrosus&account=amphibiaweb

  5. “Coastal Giant Salamander - Dicamptodon tenebrosus.” IUCN Red List of Threatened Species, 2015, https://www.iucnredlist.org/species/59081/78906025#habitat-ecology

  6. “Andrews Forest Map” HJ Andrews Experimental Forest Long-Term Ecological Research, 2017, https://andrewsforest.oregonstate.edu/data/map

  7. Gregory S. V. 2016. Aquatic Vertebrate Population Study in Mack Creek, Andrews Experimental Forest, 1987 to present. Environmental Data Initiative. https://doi.org/10.6073/pasta/5de64af9c11579266ef20da2ff32f702. Dataset accessed 12/01/2019.